University of Nevada – Reno
Computer Science & Engineering Department
Fall 2011
CPE 400 / 600
Computer Communication Networks
Lecture 3
Networking Concepts
slides are modified from J. Kurose & K. Ross
Introduction
1
Internet structure: network of networks


roughly hierarchical
at center: small # of well-connected large networks
 “tier-1” commercial ISPs (e.g., Verizon, Sprint, AT&T, Qwest,
Level3), national & international coverage
 large content distributors (Google, Akamai, Microsoft)
 treat each other as equals (no charges)
IXP
Tier-1 ISPs &
Content
Distributors,
interconnect
(peer) privately
… or at Internet
Exchange Points
IXPs
Large Content
Distributor
(e.g., Akamai)
IXP
Tier 1 ISP
Tier 1 ISP
Large Content
Distributor
(e.g., Google)
Tier 1 ISP
Introduction 1-2
Tier-1 ISP: e.g., Sprint
POP: point-of-presence
to/from backbone
peering
…
…
.
…
…
…
to/from customers
Introduction 1-3
Internet structure: network of networks
“tier-2” ISPs: smaller (often regional) ISPs
 connect to one or more tier-1 (provider) ISPs
 each tier-1 has many tier-2 customer nets
 tier 2 pays tier 1 provider
 tier-2
nets sometimes peer directly with each other
(bypassing tier 1) , or at IXP
IXP
Large Content
Distributor
(e.g., Akamai)
Tier 2
Tier 2 ISP Tier 2
ISP
ISP
IXP
Tier 1 ISP
Tier 2
Tier 1 ISP
ISP Tier 2
Tier 2
ISP
ISP
Large Content
Distributor
(e.g., Google)
Tier 1 ISP
Tier 2
ISP
Tier 2
ISP
Tier 2
ISP
Introduction 1-4
Internet structure: network of networks


“Tier-3” ISPs, local ISPs
customer of tier 1 or tier 2 network
 last hop (“access”) network (closest to end systems)
IXP
Large Content
Distributor
(e.g., Akamai)
Tier 2
Tier 2 ISP Tier 2
ISP
ISP
IXP
Tier 1 ISP
Tier 2
Tier 1 ISP
ISP Tier 2
Tier 2
ISP
ISP
Large Content
Distributor
(e.g., Google)
Tier 1 ISP
Tier 2
ISP
Tier 2
ISP
Tier 2
ISP
Introduction 1-5
Internet structure: network of networks

a packet passes through many networks from source
host to destination host
IXP
Large Content
Distributor
(e.g., Akamai)
Tier 2
Tier 2 ISP Tier 2
ISP
ISP
IXP
Tier 1 ISP
Tier 2
Tier 1 ISP
ISP Tier 2
Tier 2
ISP
ISP
Large Content
Distributor
(e.g., Google)
Tier 1 ISP
Tier 2
ISP
Tier 2
ISP
Tier 2
ISP
Introduction 1-6
Chapter 1: roadmap
1.1 What is the Internet?
1.2 Network edge

end systems, access networks, links
1.3 Network core

circuit switching, packet switching, network structure
1.4 Delay, loss and throughput in packet-switched
networks
1.5 Protocol layers, service models
1.6 Networks under attack: security
1.7 History
Introduction 7
How do loss and delay occur?
packets queue in router buffers


packet arrival rate to link exceeds output link capacity
packets queue, wait for turn
packet being transmitted (delay)
A
B
packets queueing (delay)
free (available) buffers: arriving packets
dropped (loss) if no free buffers
Introduction 8
Four sources of packet delay
transmission
A
propagation
B
nodal
processing
queueing
dnodal = dproc + dqueue + dtrans + dprop
dproc: nodal processing
 check bit errors
 determine output link
 typically < msec
dqueue: queueing delay
 time waiting at output link
for transmission
 depends on congestion level
of router
Introduction 9
Four sources of packet delay
transmission
A
propagation
B
nodal
processing
queueing
dnodal = dproc + dqueue + dtrans + dprop
dtrans: transmission delay:
 L: packet length (bits)
 R: link bandwidth (bps)
 dtrans = L/R
dtrans and dprop
very different
dprop: propagation delay:
 d: length of physical link
 s: propagation speed in
medium (~2x108 m/sec)
 dprop = d/s
Introduction 10
Nodal delay
d nodal  d proc  d queue  d trans  d prop
 dproc = processing delay
 typically a few microsecs or less
 dqueue = queuing delay
 depends on congestion
 dtrans = transmission delay
 = L/R, significant for low-speed links
 dprop = propagation delay
 a few microsecs to hundreds of msecs
Introduction
11
Caravan analogy
100 km
ten-car
caravan




toll
booth
cars “propagate” at
100 km/hr
toll booth takes 12 sec to
service car (transmission
time)
car~bit; caravan ~ packet
Q: How long until caravan
is lined up before 2nd toll
booth?
100 km
toll
booth
 time to “push” entire
caravan through toll
booth onto highway =
12*10 = 120 sec
 time for last car to
propagate from 1st to
2nd toll both:
100km/(100km/hr)= 1 hr
 A: 62 minutes
Introduction 12
Caravan analogy (more)
100 km
ten-car
caravan



toll
booth
100 km
toll
booth
cars now “propagate” at 1000 km/hr
toll booth now takes 1 min to service a car
Q: Will cars arrive to 2nd booth before all cars
serviced at 1st booth?
 A: Yes! After 7 min, 1st car arrives at second booth; three
cars still at 1st booth.
 1st bit of packet can arrive at 2nd router before packet is
fully transmitted at 1st router! (see Ethernet applet at AWL
Web site
Introduction 13



R: link bandwidth (bps)
L: packet length (bits)
a: average packet
arrival rate
average queueing
delay
Queueing delay (revisited)
traffic intensity
= La/R



La/R ~ 0: avg. queueing delay small
La/R -> 1: avg. queueing delay large
La/R > 1: more “work” arriving
than can be serviced, average delay infinite!
La/R ~ 0
La/R -> 1
Introduction 14
“Real” Internet delays and routes


What do “real” Internet delay & loss look like?
Traceroute program: provides delay
measurement from source to router along end-end
Internet path towards destination. For all i:
 sends three packets that will reach router i on path
towards destination
 router i will return packets to sender
 sender times interval between transmission and reply.
3 probes
3 probes
3 probes
Introduction 15
“Real” Internet delays and routes
traceroute: gaia.cs.umass.edu to www.eurecom.fr
Three delay measurements from
gaia.cs.umass.edu to cs-gw.cs.umass.edu
1 cs-gw (128.119.240.254) 1 ms 1 ms 2 ms
2 border1-rt-fa5-1-0.gw.umass.edu (128.119.3.145) 1 ms 1 ms 2 ms
3 cht-vbns.gw.umass.edu (128.119.3.130) 6 ms 5 ms 5 ms
4 jn1-at1-0-0-19.wor.vbns.net (204.147.132.129) 16 ms 11 ms 13 ms
5 jn1-so7-0-0-0.wae.vbns.net (204.147.136.136) 21 ms 18 ms 18 ms
6 abilene-vbns.abilene.ucaid.edu (198.32.11.9) 22 ms 18 ms 22 ms
7 nycm-wash.abilene.ucaid.edu (198.32.8.46) 22 ms 22 ms 22 ms trans-oceanic
8 62.40.103.253 (62.40.103.253) 104 ms 109 ms 106 ms
link
9 de2-1.de1.de.geant.net (62.40.96.129) 109 ms 102 ms 104 ms
10 de.fr1.fr.geant.net (62.40.96.50) 113 ms 121 ms 114 ms
11 renater-gw.fr1.fr.geant.net (62.40.103.54) 112 ms 114 ms 112 ms
12 nio-n2.cssi.renater.fr (193.51.206.13) 111 ms 114 ms 116 ms
13 nice.cssi.renater.fr (195.220.98.102) 123 ms 125 ms 124 ms
14 r3t2-nice.cssi.renater.fr (195.220.98.110) 126 ms 126 ms 124 ms
15 eurecom-valbonne.r3t2.ft.net (193.48.50.54) 135 ms 128 ms 133 ms
16 194.214.211.25 (194.214.211.25) 126 ms 128 ms 126 ms
17 * * *
* means no response (probe lost, router not replying)
18 * * *
19 fantasia.eurecom.fr (193.55.113.142) 132 ms 128 ms 136 ms
Introduction 16
Packet loss
queue (aka buffer) preceding link in buffer has
finite capacity
 packet arriving to full queue dropped (aka lost)
 lost packet may be retransmitted by previous
node, by source end system, or not at all

buffer
(waiting area)
A
B
packet being transmitted
packet arriving to
full buffer is lost
Introduction 17
Throughput

throughput: rate (bits/time unit) at which
bits transferred between sender/receiver
 instantaneous: rate at given point in time
 average: rate over longer period of time
link
capacity
that
can carry
server,
with
server
sends
bits pipe
Rs bits/sec
fluid
at rate
file of
F bits
(fluid)
into
pipe
Rs bits/sec)
to send to client
link that
capacity
pipe
can carry
Rfluid
c bits/sec
at rate
Rc bits/sec)
Introduction 18
Throughput (more)

Rs < Rc What is average end-end throughput?
Rs bits/sec

Rc bits/sec
Rs > Rc What is average end-end throughput?
Rs bits/sec
Rc bits/sec
bottleneck link
link on end-end path that constrains end-end throughput
Introduction 19
Throughput: Internet scenario
per-connection
end-end
throughput:
min(Rc,Rs,R/10)
 in practice: Rc or
Rs is often
bottleneck

Rs
Rs
Rs
R
Rc
Rc
Rc
10 connections (fairly) share
backbone bottleneck link R bits/sec
Introduction 20
Chapter 1: roadmap
1.1 What is the Internet?
1.2 Network edge

end systems, access networks, links
1.3 Network core

circuit switching, packet switching, network structure
1.4 Delay, loss and throughput in packet-switched
networks
1.5 Protocol layers, service models
1.6 Networks under attack: security
1.7 History
Introduction 21
Protocol “Layers”
Networks are complex,
with many “pieces”:
 hosts
 routers
 links of various
media
 applications
 protocols
 hardware,
software
Question:
Is there any hope of
organizing structure of
network?
Or at least our discussion
of networks?
Introduction 22
Organization of air travel
ticket (purchase)
ticket (complain)
baggage (check)
baggage (claim)
gates (load)
gates (unload)
runway takeoff
runway landing
airplane routing
airplane routing
airplane routing

a series of steps
Introduction 23
Layering of airline functionality
ticket (purchase)
ticket (complain)
ticket
baggage (check)
baggage (claim
baggage
gates (load)
gates (unload)
gate
runway (takeoff)
runway (land)
takeoff/landing
airplane routing
airplane routing
airplane routing
departure
airport
airplane routing
airplane routing
intermediate air-traffic
control centers
arrival
airport
Layers: each layer implements a service
 via its own internal-layer actions
 relying on services provided by layer below
Introduction 24
Why layering?
Dealing with complex systems:



explicit structure allows identification,
relationship of complex system’s pieces
 layered reference model for discussion
modularization eases maintenance, updating of
system
 change of implementation of layer’s service
transparent to rest of system
 e.g., change in gate procedure doesn’t affect
rest of system
layering considered harmful?
Introduction 25